i agree with deadlockedworld. Except i wouldn't use a intake fan on the bottom. I wouldn't put a fan on the bottom at all. According to your pic.. All the bottom fan is going to do is blow the hot air right back into your video card that it's trying to get rid of. (intake in the front. Out in the back and top) It's not about how much air can you get sucked into that thing. It's about how smoothly you can make the hot air exit and be replaced with fresh air.
*Edit* On second thought... An intake on the bottom might not be bad. it'll push the air from the psu and vid card up and out the back and top. But.. I still hate the idea.. thats where most of your dust and crap is going to come from. Vacuum for the loss.
won't that interfere with my videocard? And, are you sure it shod blow hot air from the case instead of getting fresh air from outside?
The CPU cooler pulling hot air from the case and blowing it out is better than heating the air before blowing it in. This is maybe a little less efficient for the CPU, but keeps the case as a whole cooler.
Bottom/side fan is optional. PSU will take up most of the space anyhow. You have filters for it that will protect PC. Heat rises, so help it rise out the top, don't fight it!